A significant but decreasing percentage of the world's chlorine and caustic soda comes from mercury amalgam cell chlor-alkali plants. These plants produce wastewater and contaminated sediments which must be treated before disposal to the environment. A common practice is to treat the wastewater with a combination of acidification and sulfidation to remove soluble mercury as mercuric sulfide. The wastewater is clarified prior to disposal while the solids containing mercuric and mercurous compounds as well as metallic mercury are disposed of in a hazardous landfill. The mercury content of the solids can vary significantly but is typically 1-6% total mercury.
Recently, the United States Environmental Protection Agency (EPA) has created a number of new rules which regulate the disposal of industrial waste streams. The rule making has identified special hazardous waste streams such as the chlor-alkali plant mercury contaminated waste-water treatment filter cake designated as K-106. The K-106 material is classified as a high mercury sub-category waste and will in the future be banned from land fill disposal.
The EPA has designated retort/roasting as the standard treatment technology (Best Demonstrated Available Technology--BDAT) for treating K-106 material. The technology is well established having been used extensively for the recovery of mercury from cinnabar ores and for the purification of contaminated mercury (triple distillation). There are, however, numerous problems associated with retorting of low concentration K-106 filter cakes. The most significant are:
1. Poor condenser recovery of mercury vapour from the low concentration retorter off-gas. PA1 2. The condensed mercury is contaminated with sulfur and carbonaceous material making further refining to triple distilled quality difficult. PA1 3. The high temperature chloride containing off-gas is highly corrosive. PA1 4. It is necessary to recover sulfur from the off-gas. PA1 5. The retort facility is expensive in terms of capital and operating cost.
U.S. Pat. No. 5,013,358, granted May 7, 1991, Ball et al., discloses and claims a method for the recovery of mercury from mercury-containing material. In that process, insoluble mercury or mercury salts in mercury-containing material are converted into a soluble form by controlled chlorination. The soluble forms of mercury in the chlorination solution are reduced with iron, preferably iron powder, to elemental mercury. After separation from the reduced solution, the solids from the reduction containing entrained mercury, are subjected to a separation procedure for the separation and quantitative recovery of substantially pure mercury. Separation by elutriation through a body of mercury is preferred. Prior to separation, the reduction solids may be kneaded for coalescence of fine mercury particles, followed by slurrying of the kneaded material. Any selenium in the reduced solution may be recovered in a reduction with a suitable reductant, preferably by adding strong sulfuric acid in the presence of the ferrous chloride formed in the preceding reduction, and excess sulfur dioxide. The process is carried out at ambient conditions, and the amount of liquid in the process is controlled. Substantially no mercury is discharged from the process in residues, or residual liquid.
The Ball et al. process utilizes chlorination in order to convert the insoluble mercury salts or mercury into soluble forms. The process does not disclose or teach concentrating mercury containing muds (K106 muds), treating the muds with acid and sodium hypochlorite in order to leach the muds, and then subsequently concentrating the mercury containing materials further and passing the materials through a second leach of acid and sodium hypochlorite. Furthermore, Ball et al. do not teach a countercurrent mud treatment process using overflow from the first and second leaches.